1. Field Of The Invention
The invention relates to a process for treating domestically-grown dates to obtain a processed date having the characteristics of Middle Eastern imported dates. The invention also relates to dates treated by the process.
2. Prior Art
Dates are grown on date palms, Phoenix dactylifera L., over a wide expanse of the semitropical middle latitudes of the eastern and western hemispheres. Dates have been used as a staple food for many years. They are of high energy value and are comprised of approximately two-thirds sugar. The date is a sticky one-seeded fruit or berry, usually oblong in shape, which varies much in shape, size, color, quality and consistency of flesh, according to the variety and the conditions of culture. The variety usually grown in California and Arizona is the Deglet Noor, a semisoft fruit with a non-sticky skin. The fruits are light brown in color but darken on storage. The dates grown in the Middle East are darker than the dates grown in the western United States. The commercial product is dehydrated, usually at a low temperature, to retain maximum flavor quality. The dried fruit is over one half sugar by weight (usually 70 to 80 percent) and contains about 2 percent each of protein, fat and mineral matter. Dates contain a high percentage of sugar either as invert or as sucrose, the level of which is maximum just before ripening. In the dry varieties such as Deglet Noor, the sugar is mostly sucrose, while the moist varieties contain invert sugar (glucose and fructose).
Maier, V.P., et al., "Studies on Domestic Dates, II. Some Chemical Changes Associated with Deterioration", pp. 322 to 328, reports that sucrose inversion, the result of invertase action, can be prevented by a mild heat treatment.
Imported dates have a darker appearance, different flavor, and a softer, chewier bite than that of U.S. domestic dates. The domestic dates have a more fibrous, tougher chew and contain more rag than imported dates. Additionally, imported dates are lower in moisture content than domestic dates and can be packaged without drying to obtain a shelf-stable product. Controlled drying is necessary to obtain a shelf-stable domestic date. A desirable characteristic of domestic dates is their uniform size and shape. The shape is retained during packaging, whereas imported dates are typically squashed upon packaging into a box. Domestic dates can basically be described as being lighter in color, less sweet, less flavorful (more fruity), and higher in moisture (therefore of borderline shelf stability at room temperature) than imported dates while domestic dates have a fibrous-type or firm texture. They are also more uniform in size and shape than imported dates.
Imported dates contain nearly all invert sugar, whereas the domestic dates contain relatively high levels of sucrose with small amounts of invert sugar. Treatment of domestic dates with enzymes to convert the sucrose into invert sugars would be uneconomical.
The nonenzymatic browning reactions are heat-induced dehydration, degradation, and condensation reactions which are accompanied with the development of yellow to brown color, and characteristic flavor. Nonenzymatic browning is usually either a caramelization or a Maillard reaction.
Johnson et al., "Encyclopedia Of Food Technology", The AVI Publishing Co., (1974), at p. 138, reports that at varying water concentrations, the browning reactions are qualitatively different. At relatively high temperatures, dehydration of the sugar itself will supply the moisture to accelerate the reaction. At lower temperatures, the air humidity is an important factor. At a given temperature, increasing the water concentration will accelerate browning until, by virtue of dilution of reactants, the rate decreases. The optimum water concentration for browning seems to be about 30 percent. The reference, at page 139, also reports that the processing and storage of susceptible products at the lowest possible temperature aids in preventing browning. Adjustment of the moisture content, either to low moisture in some cases, or to dilute solutions in others, may inhibit browning. At times, lowering the pH of the system may effectively inhibit browning during processing, after which the pH can be readjusted to the desired value.
The natural tannins of dates and the enzymatic and Maillard browning which take place during normal storage impart a brown color and a somewhat caramelized flavor. At 140.degree. F. or so a reddish-brown color as well as astringency and off-flavors occur in Deglet Noor dates. Maier et al., "Effects of heat processing on the properties of dates", Date Growers Inst. Rept., 41, (1964), pages 8 to 9. Heat processing coupled with moisture control can be used to improve the color, texture and general quantity of Deglet Noor dates, but again attempts to accelerate the process by use of temperatures in excess of 140.degree. F. may result in the development of off-flavors, off-colors and astringency. Deglet Noor dates are usually light brown in color. Page 8 of the reference reports that dark brown dates are considered to be undesirable. Page 9 of the reference reports that tough dates have invert sugar contents of 25 to 35 percent (dry weight basis), whereas tender dates have invert sugar contents above that.
U.S. Pat. No. 3,052,550 (Maier) teaches adjusting the moisture level of dates to a level of 26 to 32 percent. Then the dates are held in an atmosphere having a 70 to 80 percent relative humidity and a temperature of 130.degree. to 140.degree. F. until the majority of the sucrose in the date is converted to invert sugar. The moisture adjustment can be accomplished in any of the conventional ways, e.g., immersion in water under vacuum or exposure to a humid atmosphere. The use of steam is not disclosed by the patent. The patent states that the high-moisture content dates have a softer texture, a darkened color and increased storage stability. The sucrose in the dates is inverted to a considerable degree by the patent process. The high-moisture content dates have a lower equilibrium relative humidity than untreated dates. The high-moisture content dates can then be dried to below 23 percent moisture content (usually 18 to 22 percent) to prevent mold growth. If the dates are not so dried, a preservative is preferably added during the treatment process.
U.S. Pat. No. 2,591,213 (Stupin et al.) discloses subjecting dried dates to water vapor at 140.degree. to 155.degree. F., and further teaches that it is undesirable to exceed these temperatures (col. 1, lines 40 to 48) or else caramelization will occur in the dates. U.S. Pat. No. 2,689,182 (Richert) teaches that it is old to steam dates which have become excessively dry, and further states that high temperatures cause excessive darkening (col. 1, lines 32 to 37 and 52 to 54). Accordingly, U.S. Pat. No. 3,052,550 (Maier) is limited to an upper hydration temperature limit of 140.degree. F., (or at most 155.degree. F.). U.S. Pat. No. 3,052,550 (Maier) teaches adjusting the moisture level of dates to a level of 26 to 32 percent. The moisture adjustment can be accomplished in any of the conventional ways, e.g., immersion in water under vacuum or exposure to a humid atmosphere. The use of steam is not taught. Then the dates are held in an atmosphere having a 70 to 80 percent relative humidity and a temperature of 130.degree. to 140.degree. F. until the majority of the sucrose in the date is converted to invert sugar. U.S. Pat. No. 3,052,550 (Maier) is limited to the use of an upper temperature of no more than about 155.degree. F. in its date hydration step.
Maier et al. "Sucrose Inversion In Deglet Noor Dates And Its Processing Applications", Report Of Thirty-Eighth Annual Date Growers' Institute, (Apr. 29, 1961), pages 6 to 9, states:
"To bring about inversion in a reasonable length of time the following general procedure is suggested:
1. Hydrate dates to at least 26% moisture PA0 2. Treat with a preservative to prevent microbial growth PA0 3. Hold at 130.degree.-150.degree. F. for the length of time required to bring about the desired amount of inversion (see the following calculations) PA0 4. Dehydrate to desired final moisture content." [p. 8] PA0 a=water activity PA0 p=partial pressure of water in the food (date) PA0 p.sub.o =vapor pressure of water at same temperature PA0 ERH=equilibrium relative humidity in percent
"It is apparent that temperatures in the vicinity of 140.degree. F. are required to invert sucrose in dates rapidly enough to be considered industrially practical. The use of higher temperatures is probably possible, but undesirable changes may be more pronounced." [Emphasis supplied] [p. 8]
The teaching of the reference does not go above about 150.degree. F. The reference probably involves the same process as U.S. Pat. No. 3,052,550 (Maier) which was filed on Aug. 25, 1961, and issued on Sept. 4, 1962. U.S Pat. No. 3,052,550 states:
"Invertase is present naturally in Deglet Noor dates, but it is relatively inactive at low moistures. If sucrose is to be converted into invert sugar in a reasonable length of time, the fruit should be brought to a high moisture content in order to reactivate the invertase sufficiently." [p. 8]
Maier et al., "Effects Of Heat Processing On The Properties Of Dates", Report Of Forty-First Annual Date Growers' Institute, (Apr. 25, 1964), pages 8 and 9, further cautions against going above 140.degree. F. The reference states:
"Heat processing coupled with moisture control can be used to improve the color, texture and general quality of Deglet Noor dates, but attempts to accelerate the process by use of temperatures in excess of 140.degree. F. may result in the development of off-flavors, off-colors and astringency." [Emphasis supplied] [p. 9]
See also the last paragraph in page 8 of the reference. Johnson et al., "Encyclopedia Of Food Technology, (1974), on page 281, cited the above reference for the same temperature effect teaching.
Maier et al., "Studies On Domestic Dates II. Some Chemical Changes Associated With Deterioration", pages 322 to 328, reports some of the chemical chages, and their physical results, in Deglet Noor dates occuring during storage at 49.degree. C. (about 121.degree. F.). Caramelization is generally a relatively high-temperature chemical change in the dates which results in flavor and color changes in the date skin and flesh. U.S. Pat. No. 2,591,213 (Stupin et al.) teaches that the undesirable caramelization of relatively-dry dates (as indicated by the redness in the fleshy part of the date) occurs when the dates are subjected to water vapor at a temperature of 150.degree. F. or higher.
U.S. Pat. No. 3,365,309 (Pader et al.) teaches that fruits should not be subjected to high temperature processing as it substantially destroys or weakens tissue structures.